The present invention relates to an exposure controlling device for a lens shutter camera.
An exposure controlling device, which drive the shutter blades of a lens shutter by means of a DC motor, has been used in a conventional lens shutter camera. This type of lens shutter does not have only a function of a shutter for determining a shutter speed corresponding to a time value Tv, but also a function of an aperture stop for determining aperture area that corresponds to an aperture value Av. And therefore, both of the shutter speed and the aperture area of the shutter must be accurately controlled.
The shutter speed is controlled by using a timer. On the other hand, the conventional exposure controlling devices are classified into two types from a view point of the aperture control. In the first type, a device determines the aperture area by detecting driving amount of the DC motor by a detector such as a pulse encoder, because the driving amount corresponds to the aperture area. The pulse encoder comprises a rotation disk having slits arranged along a circumferential direction and a photointerrupter that outputs pulse signal in accordance with the rotation of the rotation disk due to the rotation of the DC motor. The first type device detects the aperture area based on the detected pulse count and controls the DC motor.
In the second type, a device controls the DC motor with detecting time period for driving the DC motor from a predetermined starting point. The time period is also related to the aperture area, the second type device detects the time period by a timer and controls the DC motor based on the detected time period.
In order to increase the accuracy of the exposure, it is required to rise a resolution of the detection, i.e., to use a fine encoder or a fine timer. However, the fine encoder requires a fine pitch rotation disk, it rises a production cost of the encoder.
On the other hand, although a fine timer is easier to construct than the fine encoder, a starting point from which the time counting starts should be defined accurately. If the starting points are different for photographs due to, for example, an unstable rotation of the DC motor, the aperture diameter cannot be determined accurately and it causes failure in photographs.